Advances in microfluidic mixers for time-resolved structural biology with X-rays

Structural biology techniques that utilize X-rays have contributed in fundamental ways to our understanding of biological macromolecules, such as proteins and nucleic acids. In addition to static structures, recent advances now allow for the observation of molecular motions using X-rays, facilitated by the many technological developments in both sources and detectors. Leveraging these advances, new approaches have been demonstrated that capture structural dynamics, sometimes with very high spatial resolution. Among the most promising are time-resolved studies, which involve triggering a reaction and capturing snapshots of reaction progression at set time delays. This review focuses on one type of time-resolved experiments, mixing experiments, in which reactants are combined within microfluidic mixers to initiate a reaction. Microfluidic mixers are extremely versatile; different designs can be optimized for various reaction types and compatibility with different structural biology techniques. When compared to other time-resolved approaches, mixing experiments are suitable for the widest range of biological applications. This review provides an overview of the current state of the field, including a review of different mixer types, and offers practical considerations for designing and performing time-resolved mixing studies with various X-ray-based structural biology methods.